ABSTRACT Thrombotic microangiopathy (TMA) is an important cause of multi-organ dysfunction syndrome (MODS) and death after hematopoietic cell transplantation (HCT). TMA with MODS occurs in about 11-15% of children after HCT. Mortality with TMA and MODS is about 80%, both in the literature and in our own experience. We have shown that patients with TMA develop MODS through endothelial damage mediated by activation of complement, and have identified complement activation as a poor prognostic marker in TMA. Most importantly, we have shown in a single institution study that complement blockade with eculizumab improves survival. We hypothesize that functional complement studies performed prior to and during HCT will identify patients with increased susceptibility to severe TMA during HCT, and will provide novel pre-transplant screening tools for TMA risk stratification. We also hypothesize that early intervention with eculizumab will double survival in HCT recipients with high risk TMA and MODS, as compared to historical untreated controls. An important goal of this proposal is to identify biological markers that define high-risk patients prior to proceeding to HCT. Our published complement gene sequencing studies indicate that susceptibility to TMA is polygenic, with disease only manifesting under the extreme stress of the HCT process. We found that multiple gene variants modify TMA susceptibility, making pre-transplant genotyping in all patients an impractical approach to identifying high-risk children. In our first specific aim we will test strategies using functional protein-based assays that will allow us to both identify highly susceptible children prior to HCT, and identify the onset of TMA earlier than we currently do, to allow prophylactic and/or earliest possible therapy in future clinical trials. Our first strategy is to test resting state complement activation in serum from 80 children (40 with and 40 without TMA) prior to starting transplant using an endothelial complement deposition assay and a modified Ham?s test, to identify patients at high risk to activate complement during HCT. Our second strategy is to identify endothelial injury occurring after HCT at the earliest possible time by measuring ST2 and NETS formation in 200 children systematically phenotyped for TMA at weekly intervals after HCT. Early identification of endothelial injury will allow intervention before organ damage occurs. In addition, we will perform RNAseq analysis of peripheral blood mononuclear cells from 40 children with TMA at the time of disease, and after resolution of TMA, to identify undescribed dysregulated pathways, and novel potentially druggable targets. In our second aim we will establish an eculizumab treatment regimen that can be generalizable to multiple sites and does not require time-intensive individual pharmacokinetic and pharmacodynamic (PK/PD) monitoring. In preparation for this study we established prospective TMA screening strategies, determined high-risk disease markers, and performed eculizumab PK/PD studies in children receiving HCT. We will treat 21 patients at four major pediatric centers as part of multi-institutional study to establish exportability to other institutions. The regimen is expected to provide satisfactory outcomes for a large majority of children without highly specialized monitoring. These data will be of immediate clinical importance, guiding modification of transplant strategies, TMA screening, and early interventions to improve short and long term transplant outcomes.